He is warning gardeners to take care with bagged potting mix and compost to avoid the life-threatening disease.

“Gardeners are at particularly high risk of catching Legionnaires' disease as the bacteria thrives in bags of potting mix and compost,” says Dr Humphrey.

With an incubation period of up to two weeks, it’s possible that up to fifteen cases may have occurred during Labour weekend last year, Canterbury’s traditional start to the gardening season.

Dr Humphrey says there are five simple actions gardeners should do to avoid getting legionnaires’:

Open potting mix or compost carefully – use scissors instead of ripping the bag.

Wear a well-fitting disposable face mask and gloves, and remember not to touch your mask when gardening.

Reduce dust by dampening down potting mix or compost with a sprinkle of water.

Work with potting mix or compost in a well-ventilated outdoor area.

Thoroughly wash your hands after handling potting mix or compost.

“Legionnaires’ is a very serious illness and these simple actions can be lifesaving.”

Last year 62 Cantabrians were hospitalised with Legionnaires’. Thirteen of these patients spent extended periods of time in ICU, including one patient who was in ICU for 42 days.

With the average cost of treating someone with an infectious disease in ICU estimated at close to $5,000 per day, Dr Humphrey says Legionnaires’ is costing our health system hundreds of thousands of dollars.

“The number of cases last spring was around three times more than average. We don’t know why the bacteria was particularly virulent then, but one theory is that the warmer than usual spring led to potting mix and compost heating up in the bag more than usual, creating a perfectly warm and moist environment for the bacteria to thrive.”

“With NIWA predicting another warm spring and summer, it’s extremely important to take care when using bagged potting mix and compost.”

The illness may be mild but can sometimes be fatal. It is more common in older people, particularly if they smoke, have poor immunity or a chronic illness. Symptoms include dry coughing, high fever, chills, diarrhoea, shortness of breath, chest pains, headaches, excessive sweating, nausea, vomiting and abdominal pain.

Anyone who gets these symptoms should see their general practice team right away and let them know if you have been handling potting mix or compost.